It is well known in the art that cellulosic products such as untreated paper are permeable to water and other aqueous and non-aqueous liquids. Thus, it has been common practice to coat or treat paper and other cellulosic products with various coatings, such as waxes, polymers, fluorocarbons and the like in order to impart barrier properties and resistance to various liquids, such as water, oil, grease, solvents, etc. Such coated or treated paper, paperboard and boxboard substrates are used for food packaging and food wrappers, food containers, and various other paper substrates that contact foods. Recent cellulosic substrate grade developments for packaging or holding high fat/oil content foods, such as fast food and microwave applications, require the coating on the cellulosic substrate to prevent fluids, such as hot oils and/or fats, from penetrating through the cellulosic substrate and staining the outside of the package. Further, such coatings must also prevent such fluid penetration through the cellulosic substrates for the shelf life of the packaged foods, which can experience elevated temperatures in warehouses and/or shipping vessels.
Water repellant paper products, such as paperboard for use in packaging meats, fruits and vegetables, coated with polyethylene and waxes are difficult to repulp. The coatings tend to hold the fibers together. Often the particles of the coatings stick to paper mailing equipment, wire presses, and dryers. In addition, many of the barrier coated paper products for packing food products and the like become tacky causing stacked or contiguous sheets of coated board to block or stick together.
Cellulosic substrates utilized for food packaging, such as fast food and microwave applications, need to be flexible since the material is often folded, scored or wrapped during conversion into the final container. Barrier coatings for these cellulosic substrates need to protect the cellulose fibers at these folds, scores or wraps to prevent wicking of oils, fats and other liquids through the substrate. Fluorocarbons accomplish this by lowering the surface energy of the fibers. Most polymer emulsion coatings fail to protect the fold or scoreline because the polymer films break when subjected to such stresses. The polymer emulsion coatings of this invention are unique in that their flexibility allows the coating to stretch over the fold or scoreline and prevent liquids from wicking through the substrate.
Representative patents illustrating the prior art in the field of barrier coated paper products include the following:
U.S. Pat. No. 5,897,411 discloses paper and paperboard for use in packaging operations, e.g., food packaging which have been coated with a repulpable moisture vapor barrier layer comprising a resin latex and a hydrophobic component with a platelet structure. Examples include polystyrene, styrene-acrylonitrile, carboxylated butadiene-styrene, ethylene/vinyl chloride and the like. Examples of hydrophobic components with a platelet structure include mica, talc, silica, etc.
U.S. Pat. No. 6,066,379 discloses a repulpable water repellant paperboard incorporating an aqueous coating containing a polymer matrix/wax/pigment mixture as a barrier as an improvement to wax-coated board. The water-repellant coating of film includes pigments, wax, and a polymer matrix of polymer chains ionically cross-linked through pendent carboxylic acid, such as carboxylated butadiene-styrene latexes.
U.S. Pat. No. 5,876,815 discloses a laminate product having both improved grease, oil, wax and solvent repellency and improved glueability and printability properties. The laminate comprises a paper substrate, i.e., the top side of at least one layer of a fluorine containing polymer moiety on at least one surface of the substrate and at least one layer comprising a latex on the at least one layer of a fluorine containing polymer moiety layer. Examples of latexes include styrene-acrylic copolymers, ethylene/vinyl acetate (Airflex 100HS), ethylene/vinyl chloride, vinyl acetate/acrylic etc.
WO 02/14426 A1 discloses grease resistant paper products incorporating a barrier formulation that does not contain fluorochemicals. The formulation is also useful as a coating or treatment for providing release properties to an underlying substrate without the use of silicone. The formulation contains polyvinyl alcohol and a fatty acid melamine and paraffin wax.
U.S. Pat. No. 5,989,724 discloses a recyclable and repulpable coated paper stock coated on one surface with a base coat and at least one additional coat over the base coat. Both coats are based upon polymers such as acrylic, ethylene-vinyl acetate, polyvinyl acetate, and the like. Unlike polyethylene films and wax coatings, the coatings described are repulpable.
U.S. Pat. No. 3,436,363 discloses a batchwise and a continuous process for the polymerization of ethylene in the presence of a polymerizable ethenoid unsaturated mono or polybasic carboxylic acid or sulfonic acid and water. Representative polymerizable ethenoid acids include acrylic, methacrylic, crotonic, itaconic, and the like. The solids level of the polymer emulsions is about 25 to 30% by weight. The emulsions are reported to be useful in paper coating, textile finishing, polishes, and surface coatings.